Screw compressors are commonly used in air conditioning and refrigeration systems. It is well known that screw compressors, along with rotary vane compressors and the more recently introduced scroll compressors, are constant volume ratio compressors. For screw compressors, the internal volume ratio, V.sub.i, is defined as the volume of uncompressed vapor in one groove of the compressor before compression begins divided by the volume of the compressed vapor in the groove just prior to the uncovering of the discharge port. This ratio is fixed during the manufacture of the machine by the size of the compressor grooves and the location of the suction and discharge ports of the compressor.
Since screw compressors are constant volume ratio machines, they are also constant pressure ratio, P.sub.i, machines. Assuming isentropic compression, the volume ratio is related to the pressure ratio by the following equation: ##EQU1## where: k=the isentropic exponent of the refrigerant being used
P.sub.d =the internal discharge pressure of the screw compressor PA1 P.sub.s =the suction pressure of the screw compressor
As can be seen from the above relationship, for a given refrigerant, the screw compressor internal discharge pressure, P.sub.d, is dependant only on the built in volume ratio, V.sub.i and suction pressure, P.sub.s. Thus, in systems utilizing a compressor with a constant internal volume ratio and where the suction pressure is held constant, the internal discharge pressure will also remain constant.
It is important to note that while the relationship shown above for V.sub.1 and P.sub.i is correct for isentropic compression, it is recognized that screw compressors do not perform in a pure isentropic fashion. The vapor being compressed within the grooves of the compressor is cooled to some degree by the oil injected injected into the compressor. In addition, the grooves of the compressor are not perfectly sealed which allows a small portion of the refrigerant to blow-through, or leak out of, the grooves during compression. As a result, the ideal pressure ratio is not achieved. This change of pressure can be determined from the adiabatic compressor efficiency of the compressor and a correction factor applied to obtain the "ideal" pressure ratio.
It is well known that for the most economical operation, the internal discharge pressure of the screw compressor should equal the pressure of the refrigerant within the line into which the screw compressor discharges. This is referred to as ideal compression. However, in many cases where the internal discharge pressure remains relatively constant, ideal compression is not achieved due to changes in the condensing pressure and hence, the discharge line pressure. The discharge line pressure can be considered equal to the condensing pressure in most applications because the only difference in these two pressures is the relatively small pressure loss which occurs in the line between the outlet of the compressor and the inlet of the condenser. As a result, the discharge line pressure will vary directly with the condensing pressure.
The condensing pressure at which a condenser will operate depends upon a number of factors such as the design conditions for which the condenser was selected, the actual conditions at which the condenser is operating, and whether the condenser is operating at full or partial capacity. In many cases, condenser operations in refrigeration and air conditioning systems are operated at full capacity at all times. In these situations, the pressure at which the condenser operates will fluctuate as changes occur in the ambient conditions such as outside air temperature or humidity. Because of these condensing pressure fluctuations, refrigeration or air conditioning systems utilizing screw compressors typically operate where the internal discharge pressure of the compressor does not equal the condensing or discharge line pressure resulting in a condition of either "over-compression" or "under-compression".
In the under-compression case, the internal discharge pressure is less than the discharge line pressure. Energy is wasted because the compressor must work against this higher pressure from the time the discharge port is uncovered until all gas is pushed out of the cavity. In the over-compression case, the internal discharge pressure is greater than the discharge line pressure. Energy is wasted in this case when the condenser needlessly operates at full capacity, thereby keeping the discharge line pressure low, when operation at less than full capacity would be sufficient.